In a piezoelectric ink jet printer, a print head includes a large number of ink chambers, each of which is in fluid communication with an orifice and with an ink reservoir. At least one wall of the ink chamber is coupled to a piezoelectric material. When actuated, the piezoelectric material deforms. This deformation results in a deformation of the wall, which in turn launches a pressure wave that ultimately pushes ink out of the orifice while drawing in additional ink from an ink reservoir.
To provide greater density variations on a printed image, it is often useful to eject ink droplets of different sizes from the ink chambers. One way to do so is to sequentially actuate the piezoelectric material. Each actuation of the piezoelectric material causes a bolus of ink to be pumped out the orifice. If the actuations occur at a frequency that is higher than the resonant frequency of the ink chamber, successive boluses will arrive at the orifice plate before the first bolus has begun its flight to the substrate. As a result, all of the boluses merge together into one droplet. The size of this one droplet depends on the number of times actuation occurs before the droplet begins its flight from the orifice to the substrate. An ink jet printer of this type is disclosed in co-pending application Ser. No. 10/800,467, filed on Mar. 15, 2004, the contents of which are herein incorporated by reference.